This study presents the porosity and corresponding specific surface area evolution of pharmaceutical tablets vs. compaction pressure. A model based on quasi-chemical treatments and population balances is proposed to understand these evolutions. The main phenomena, namely fragmentation and plastic deformation, which occur in pharmaceutical powder compaction, are taken into account to explain the conjugate evolution of these macroscopic properties. These phenomena are quantified through the different parameters of the model. Experimental results on tablettose, saccharose and ketoprofen tablets show a fair agreement with theoretical results, and allow us to classify the fragmentation and the plastic deformation abilities of the products.